1. Field of the Invention
The present invention relates to a reticle, a pattern transferred by a reticle, and a correction method. More particularly, the present invention relates to a reticle, a pattern transferred by a reticle, and a correction method employed in an exposure process for a semiconductor wafer.
2. Description of the Background Art
An exposure process is conventionally known as one manufacturing step for ICs and LSIs. In this exposure process, deviation in the rotation of an exposure region (chip rotation) and deviation in the magnification of an exposure region (chip magnification) are encountered originating from the semiconductor wafer and exposure device (stepper).
Detection of such chip rotation and chip magnification is conventionally carried out as set forth in the following. An underlying pattern formed in advance is exposed and developed. A further measurement pattern is formed above that underlying pattern. The amount of deviation between that measurement pattern and the underlying pattern is gauged. However, it was difficult to gauge the chip rotation and the chip magnification in the first exposure step (first step) where there is no underlying pattern on the wafer according to conventional art. There was a need for a measurement pattern that allows gauging of the chip rotation and chip magnification with only a wafer subjected to exposure and development without requiring an underlying pattern.
In the past years, the demand for overlay accuracy is critical. It is now necessary to correct the exposure condition after gauging deviation in chip rotation and chip magnification for one wafer in one lot to eliminate variation in the exposure condition for the first step for other wafers in the same lot. It has also become necessary to render uniform the exposure condition of the first step between a plurality of steppers. When the first step is carried out using a plurality of steppers, there will be a mixture of semiconductor wafers subjected to the first step of exposure with the plurality of steppers at the steps subsequent to the first step. In order to improve the overlay accuracy between each step, the plurality of steppers must have the exposure condition of the first step set equal. In order to meet these requirements, variation in chip rotation and variation in chip magnification must be calculated independently for each stepper to correct the exposure condition of each stepper according to the calculated result.
FIGS. 12 and 13 are plan views showing a conventional measurement pattern (fiducial mark). This fiducial mark is disclosed in Japanese Patent Laying-Open No. 6-324475. More specifically, FIG. 12 is a plan view showing a conventionally proposed reticle, and FIG. 13 is a plan view showing a fiducial mark formed on a wafer using the reticle of FIG. 12.
Referring to FIG. 12, circuit pattern region 25 of a rectangular configuration is formed at a predetermined region of a transparent substrate 22 by the conventional reticle. A wafer alignment mark 26 is formed in an outer perimeter area of circuit pattern region 25. Also, respective fiducial marks 27 are formed at the four corners outer of circuit pattern region 25. A light blocking region 24 is formed so as to surround respective fiducial mark 27. Four reticle alignment marks 23 are formed at the outer side of light blocking region 24.
By transferring the pattern of the used reticle of FIG. 12 on a wafer, a fiducial mark as shown in FIG. 13 is obtained. The fiducial mark of FIG. 13 shows a specific plane configuration of fiducial mark 27 shown in FIG. 12. Referring to FIG. 13, the fiducial mark formed on the wafer includes a first fiducial mark 28 and a second fiducial mark 29. First fiducial mark 28 includes a projection portion 30. Second fiducial mark 29 includes a projection portion 31. The pitch of each projection of projection portion 30 is set slightly offset the pitch of the projection of projection portion 31.
In operation, exposure is carried out using the conventional reticle shown in FIG. 12. After a development process, first fiducial mark 28 and second fiducial mark 29 shown in FIG. 13 are formed on an exposure region on a wafer. The amount of deviation is gauged by obtaining the difference between distances a and b between projection portion 30 of first fiducial mark 28 and projection portion 31 of second fiducial mark 29.
However, the specific method of detecting the deviation and correction method thereof according to the conventional fiducial marks shown in FIGS. 12 and 13 are not disclosed in Japanese Patent Laying-Open No. 6-324475. The procedure of calculating deviation in chip rotation and chip magnification independently to carry out correction is also not disclosed. It was therefore difficult to employ the conventional art shown in FIGS. 12 and 13 for correcting the chip rotation and chip magnification at the time of exposure with a stepper.
The fiducial mark shown in FIG. 13 induces the possibility of deformation in the fiducial mark when there is aberration. It was difficult to carry out accurate gauging in such a case. According to the first and second fiducial marks 28 and 29 of FIG. 13, there may be a case where the influence from aberration differs between the fiducial mark and the circuit pattern region depending upon the pattern type of the circuit pattern region. In such a case, usage of the fiducial mark of FIG. 13 as measurement means for alleviating the influence of aberration upon the circuit pattern region is inappropriate.
Furthermore, the fiducial mark shown in FIG. 13 has the disadvantage that a larger occupying area is required since first and second fiducial marks 28 and 29 do not completely overlap each other. There was a problem that the exposure region of the stepper cannot be used effectively.
According to the conventional fiducial mark shown in FIG. 13, second fiducial mark 29 is formed above first fiducial mark 28 previously formed as an underlying pattern to detect the deviation between the first and second fiducial marks 28 and 29. Therefore, there was a problem that it was difficult to use such conventional marks when exposure is to be carried out at the first step.